Printed circuits often have to meet certain requirements. They should have the capability of being wettable by solder so that a good bond between a circuit and its lead is made. Because they must be handled, the printed circuits should have good adhesion to a substrate. The circuit should have good electrical conductivity. Moreover, fabrication of a device using the printed circuit should facilitate manufacturing, for example, a process eliminating plating the circuit prior to soldering.
There are many processes known for making printed circuits or printed circuit boards. However, each process that is discussed below is disadvantageous in one or more respects.
U.S. Pat. No. 4,396,666 describes a method of painting an epoxy composition containing 65-85 percent by weight of a solderable metal powder onto a substrate and then slightly pyrolytically decomposing the cured epoxide to expose sufficient metal for soldering. This method suffers from some obvious deficiencies. One apparent deficiency is that the decomposition of the resin can be irregular, and the exposed surface of the circuit will also be irregular. Another deficiency is that expulsion of volatile matter from the circuit will open up the surface to crater formations, pits and the like, which could adversely affect the integrity of the circuit. Also, pyrolytic decomposition can attack the integrity of the film and especially the binder portion. Thus, adhesion of metal particles to the substrate would be suspect after decomposition. Because pyrolytic temperatures are great, at least 200.degree. C., substantial damage to the substrate would occur if it were made of a thermoplastic material.
Japanese patent application 56,357/73 (published Jan. 26, 1981, Publication No. 2,435/81) describes printing a circuit pattern with a conductive ink on a transfer paper and then bringing the printed surface of the transfer paper into contact with a board for instant thermocompression bonding. The pattern is formed by a screen printing process onto the transfer paper. The application cites the following advantages for this process, including simplicity, accuracy, uniformity, non-polluting steps, recycling of poorly formed boards and manufacturing versatility. In attempts to follow the procedure of the Japanese patent, there are problems in obtaining transfer of printed circuit to dielectric surface; and resultant products can be lacking in desired parameters of conductivity, solderability and/or plateability.
The ink used in the Japanese application contained silver suspended in a solvent containing solution of a thermoplastic copolymer. Example 1 uses a vinyl chloride-vinylacetate copolymer and Example 2 uses an "acrylic resin" with no description of its composition. In Example 1, the "silver powder" comprises 89.47% of the solids weight of the composition; and in Example 2, it comprised 88.09% of the solids weight of the composition. The reference states that good plateability, easy soldered circuits and good adhesion are obtained. However, the reference fails to state the kind of circuit board that was used.
Though the technique of the Japanese application may be capable of transfer of the circuit to the board, the technique is disadvantageous. The amount of thermoplastic resin in the ink can cause the resin to coat the metal sufficiently to adversely affect soldering, wettability and electroless plating of the circuit. In addition, use of a thermoplastic resin limits effective soldering of the circuit and significantly limits the technology to soldering techniques that are not current state-of-the-art methods.
Another technique is the dusting method of U.S. Pat. No. 4,327,124. There a phenolic thermosetting resin (B-stage) is loaded with copper powder to achieve 70-75% loading. This is silk screened onto a board to form the circuit pattern. Then, copper powder is dusted onto the print of wet resin film. The film is cured and treated with solder. As with other techniques, disadvantages exist.
Because dusting of copper powder is an awkward process which gives no assurance of uniform metal distribution, the use of copper powder per se is a questionable selection in view of its propensity to form a non-conductive oxide as it ages. Metal powders of metals which form oxides more readily form oxides because of increased surface area provided by the powder.
Other techniques are known where 90% by weight or slightly higher of silver particle-containing inks are known for certain circuit pattern uses. See U.S. Pat. No. 4,264,477.
Certain methods of applying circuit elements to a substrate are known in the art. U.S. Pat. No. 4,329,779 teaches a method of applying circuit elements by direct contact transfer. The reference shows laminating elements of a circuit on a carrier layer by applying adhesive between the circuit and a substrate without compaction and without using heat sensitive bonding pastes or inks. The circuit element is formed by thin film fabrication techniques. Thus, the patent relates to a different technology than that utilized and discovered by applicant. This involves deposition in a vacuum of thin films of conductive, resistive or insulating materials by sputtering, evaporation, or chemical vapor deposition. According to the reference, the technique differs from thick film technology which can involve the use of pastes or inks like the present process. Overall, the reference shows laminating elements of a circuit on a carrier layer by applying adhesive between the circuit and a dielectric surface, bringing these together under pressure, bonding to form a composite structure and subsequently removing the carrier layer. Bonding in the reference is preferably by ultrasonic bonding.
U.S. Pat. No. 4,029,845 relates to a thermosetting resin in the use of heat and pressure to form a composite circuit board. The reference only discloses forming the baseboard and does not teach forming printed circuit elements on that baseboard. The reference refers to an additive process for manufacturing a printed circuit board without clear explanation of that teaching.
U.S. Pat. No. 4,180,608 teaches heat and pressure used to form a composite printed circuit board. However, the reference uses a carrier layer for resin but not for particle loaded ink as in the present invention. A lamination is formed as taught in the art.
U.S. Pat. No. 4,373,091 teaches thick film pattern forming methods. The reference discusses applying the paste directly to printed circuit boards. However, it is conventional to apply conductive inks and inks which can be plated to printed circuit boards in the form of circuit patterns.
U.S. Pat. No. 3,950,200 describes a method of fabricating a thermal printing head by printing a circuit pattern of resistive elements with electric conductors on one surface of a flat paper sheet, covering the circuit pattern with a carrier layer of resin, peeling off the flat paper sheet and placing the remaining carrier layer carrying the circuit pattern over a surface of insulating substrate. The circuit pattern is laid downward, and the carrier layer is heated. This causes the carrier layer to be burned off and causes the circuit pattern to adhere to the substrate. The method further requires use of an adhesive adjacent to the paper sheet. The circuit pattern is printed on the adhesive. A carrier layer then is formed on the circuit pattern, and the paper sheet is peeled off. The method is used with a ceramic substrate. The process is cumbersome and duplicative.
U.S. Pat. Nos. 4,424,091 and 4,464,420 teach conventional plating subsequent to producing a conductive pattern. Similarly, electroless plating is known in the art, that is, deposition of a metal coating by immersion of a metal or a non-metal in a suitable bath containing a chemical reducing agent. These plating techniques are utilized by the prior art to effect soldering of connections to the printed circuit. This additional processing step complicates processing and manufacturing of printed circuit articles. While such a process is necessary in the prior art, it is advantageously avoided by applicants' process. This achievement by applicants points up one of the novel aspects of the present invention to be discussed in greater detail below.
However, none of such known techniques obtain the advantages of the present invention. All the preceding references are hereby incorporated by reference.